Vacuolar Na+/H+ antiporter cation selectivity is regulated by calmodulin from within the vacuole in a Ca2+- and pH-dependent manner

The selective movement of ions between intracellular compartments is fundamental for eukaryotes. Arabidopsis thaliana Na+/H+ exchanger 1 (AtNHX1), the most abundant vacuolar Na+/H+ antiporter in A. thaliana, has important roles affecting the maintenance of cellular pH, ion homeostasis, and the regulation of protein trafficking. Previously, we have shown that the AtNHX1 C-terminal hydrophilic region localized in the vacuolar lumen plays an important role in regulating the antiporter's activity. Here, we have identified A. thaliana calmodulin-like protein 15 (AtCaM15), which interacts with the AtNHX1 C terminus. When expressed in yeast, AtCaM15 is localized in the vacuolar lumen. The transient expression of AtCaM15 in Arabidopsis leaf protoplasts showed that AtCaM15 is present in the central vacuole. The binding of AtCaM15 to AtNHX1 was Ca2+- and pH-dependent and decreased with increasing pH values. Our results also show that the binding of AtCaM15 to AtNHX1 modified the Na+/K+ selectivity of the antiporter, decreasing its Na+/H+ exchange activity. Taken together, the presence of a vacuolar calmodulin-like protein acting on the vacuolar-localized AtNHX1 C terminus in a Ca2+- pH-dependent manner suggests the presence of signaling entities acting within the vacuole.